CHAPTER 4 :JFET

Junction Field Effect Transistor

Introduction (FET) Field-effect transistor (FET) are important

devices such as BJTs Also used as amplifier and logic switches Types of FET:

MOSFET (metal-oxide-semiconductor field-effect transistor)

Depletion-mode MOSFET JFET (junction field-effect transistor)

What is the difference between JFET and MOSFET?

Current-controlled amplifiers

Voltage-controlled amplifiers

High input impedance (M) (Linear AC amplifier system)

Temperature stable than BJT Smaller than BJT Can be fabricated with fewer processing BJT is bipolar – conduction both hole and

electron FET is unipolar – uses only one type of current

carrier Less noise compare to BJT Usually use as logic switch

Introduction.. (Advantages of FET)

Disadvantages of FET

Easy to damage compare to BJT ???

Junction field-effect transistor (JFET)

There are 2 types of JFET n-channel JFET p-channel JFET

Three Terminal Drain – D (Saliran) Gate -G (Get) Source – S (Punca)

Junction field-effect transistor..

N channel JFET: Major structure is n-type material (channel)

between embedded p-type material to form 2 p-n junction.

In the normal operation of an n-channel device, the Drain (D) is positive with respect to the Source (S). Current flows into the Drain (D), through the channel, and out of the Source (S)

Because the resistance of the channel depends on the gate-to-source voltage (VGS), the drain current (ID) is controlled by that voltage

N-channel JFET

N-channel JFET..

P channel JFET: Major structure is p-type material

(channel) between embedded n-type material to form 2 p-n junction.

Current flow : from Source (S) to Drain (D)

Holes injected to Source (S) through p-type channel and flowed to Drain (D)

P-channel JFET

P-channel JFET..

Water analogy for the JFET control mechanism

JFET Characteristic Curve

To start, suppose VGS=0 Then, when VDS is increased, ID increases. Therefore,

ID is proportional to VDS for small values of VDS

For larger value of VDS, as VDS increases, the depletion layer become wider, causing the resistance of channel increases.

After the pinch-off voltage (Vp) is reached, the ID becomes nearly constant (called as ID maximum, IDSS-Drain to Source current with Gate Shorted)

ID versus VDS for VGS = 0 V.

JFET Characteristic Curve

JFET for VGS = 0 V and 0<VDS<|Vp|

Channel becomes narrower as VDS is increased

Pinch-off (VGS = 0 V, VDS = VP).

Application of a negative voltage to the gate of a JFET.

JFET Characteristic Curve..

For negative values of VGS, the gate-to-channel junction is reverse biased even with VDS=0

Thus, the initial channel resistance is higher (in which the initial slope of the curves is smaller for values of VGS closer to the pinch-off voltage (VP)

The resistance value is under the control of VGS

If VGS is less than pinch-off voltage, the resistance becomes an open-circuit ;therefore the device is in cutoff (VGS=VGS(off) )

The region where ID constant – The saturation/pinch-off region

The region where ID depends on VDS is called the linear/triode/ohmic region

n-Channel JFET characteristics curve with IDSS = 8 mA and VP = -4 V.

JFET Characteristic Curve

p-Channel JFET

p-Channel JFET characteristics with IDSS = 6 mA and VP = +6 V.

Characteristics for n-channel JFET

P

+

+

+

Characteristics for p-channel JFET

Operation of n-channel JFET JFET is biased with two voltage sources:

VDD

VGG

VDD generate voltage bias between Drain (D) and Source (S) – VDS

VDD causes drain current, ID flows from Drain (D) to Source (S)

VGG generate voltage bias between Gate (G) and Source (S) with negative polarity source is connected to the Gate Junction (G) – reverse-biases the gate; therefore gate current, IG = 0.

VGG is to produce depletion region in N channel so that it can control the amount of drain current, ID that flows through the channel

Transfer Characteristics

The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT. In BJT:

IC= IB

which is defined as the relationship between IB (input current) and IC (output current).

Transfer Characteristics..

In JFET, the relationship between VGS (input voltage) and ID (output current) is used to define the transfer characteristics. It is called as Shockley’s Equation:

The relationship is more complicated (and not linear)As a result, FET’s are often referred to a square law devices

2GS

D DSSP

V I = I 1 -

VVP=VGS (OFF)

Defined by Shockley’s equation:

Relationship between ID and VGS. Obtaining transfer characteristic curve axis

point from Shockley: When VGS = 0 V, ID = IDSS

When VGS = VGS(off) or Vp, ID = 0 mA

)(

2

)(

1 offGSP

offGS

GSDSSD VV

V

VII

Transfer Characteristics…

Transfer Characteristics

JFET Transfer Characteristic Curve JFET Characteristic Curve

Exercise 1

DGS P

DSS

I V = V 1 -

I

2GS

D DSSP

V I = I 1 -

V

VGS ID0 IDSS

0.3Vp IDSS/2

0.5Vp IDSS/4

Vp 0 mA

Sketch the transfer defined byIDSS = 12 mA dan VGS(off) = - 6

Exercise 1

DGS P

DSS

I V = V 1 -

I

Sketch the transfer defined by IDSS = 12 mA dan VGS(off) = Vp= - 6

IDSS

IDSS/2

IDSS/4

2GS

D DSSP

V I = I 1 -

VVGS =0.3VP

VGS =0.5VP

Answer 1

Exercise 2

DGS P

DSS

I V = V 1 -

I

2GS

D DSSP

V I = I 1 -

V

VGS ID0 IDSS

0.3Vp IDSS/2

0.5Vp IDSS/4

Vp 0 mA

Sketch the transfer defined byIDSS = 4 mA dan VGS(off) = 3 V

Exercise 2

DGS P

DSS

I V = V 1 -

I

Sketch the transfer defined by IDSS = 4 mA dan VGS(off) = 3V

2GS

D DSSP

V I = I 1 -

V

VGS =0.5VP

VGS =0.3VP

VP

IDSS

IDSS/2

IDSS/4

Answer 2Answer 2

DC JFET Biasing

Just as we learned that the BJT must be biased for proper operation, the JFET also must be biased for operation point (ID, VGS, VDS)

In most cases the ideal Q-point will be at the middle of the transfer characteristic curve, which is about half of the IDSS.

3 types of DC JFET biasing configurations : Fixed-bias Self-bias Voltage-Divider Bias

Fixed-bias

VDS

+_

VGG

VGS_

RD

VDD

RG

+C1

C2

Fixed-bias

+

Vin

_

+

Vout

_

+ Use two voltage sources: VGG, VDD

VGG is reverse-biased at the Gate – Source (G-S) terminal, thus no current flows through RG (IG = 0).

Fixed-bias.. DC analysis

All capacitors replaced with open-circuit

VDS

+_

VGG

VGS_

RD

VDD

RG

+

Loop 1

Fixed-bias…

1. Input Loop By using KVL at loop 1:

VGG + VGS = 0 VGS = - VGG

For graphical solution, use VGS = - VGG to draw the load line

For mathematical solution, replace VGS = -VGG in Shockley’s Eq. ,therefore:

2. Output loop- VDD + IDRD + VDS = 0

VDS = VDD – IDRD

3. Then, plot transfer characteristic curve by using Shockley’s Equation

2

)(

2

)(

11

offGS

GGDSS

offGS

GSDSSD V

VI

V

VII

Example : Fixed-bias

2GS

D DSSP

V I = I 1 -

V

Determine the following network:

1. VGSQ

2. IDQ

3. VD

4. VG

5. VS

Mathematical Solutions

GSQ GGV = - V = - 2

2 2GS

DQ DSSP

2

V - 2 I = I 1 - = 10mA 1 -

V -8

= 10mA 0 5.6.75 25mA

DS DD D DV = V - I R = 16 - 5.625mA 2k

= 16V -11.25V = 4.75V

Graphical solution for the network

GSQ GGV = - V = - 2

DS 4V = .75V

D

G

S

V =

V =

4.75V

- 2V

V = 0V

Draw load line for:

Self-bias

Using only one voltage source

DC analysis of the self-bias configuration.

G G GRG

RG

Since I 0A, V I R

thus V 0A,

Q point for VGS

Graphical Solutions:Defining a point on the self-bias line.

VGS ID0 IDSS

0.3Vp IDSS/2

0.5Vp IDSS/4

Vp 0 mA

Graphical Solutions: Sketching the self-bias line.

D DSS

GS D S

DSS S

I = I 2

V = -I R

I R = -

2 DS DD D S DV = V - I R + R

S D SV = I R

Mathematical Solutions:

Replace in the Shockley’s Equation:

By using, quadratic equation and formula, choose value of ID that relevant within the range (0 to IDSS): nearly to IDSS/2

Find VGS by using ;also choose VGS that within the range (0 to VP)

2

)(

2

)(1

;

1

P

SDDSSD

offGSPP

GSDSSD

V

RIII

therefore

VVV

VII

Example : Self-bias configuration

GSQ

DQ

D

G

1. V

2. I

Det

3. V

e

rmine the following for

4. V

the network

5. Vs

Graphical Solutions:

Sketching the transfer characteristics curve

Vgs ID

0 IDSS

0.3Vp IDSS/2

0.5Vp IDSS/4

Vp 0 mA

Sketching the self-bias line

D GS

D GS

When I = 4mA, V =

When I = 8mA, V

- 4V

= - 8V

Graphical Solutions: Determining the Q-point

Q-point

IDQ=2.6mAVGSQ=-2.6mV

Mathematical Solutions

VVandmAIchoosetherefore

VV

kmAkmA

RIVRIV

mAImAI

IkI

kIII

MIkIkIm

kIm

kImI

V

RII

RIVrecallV

VII

GSD

SDGSSDGS

DD

DD

DDD

DDD

DDD

P

SDDSS

SDGSP

GSDSSD

6.2588.2;

6.29.13

)1(588.2)1(9.13

588.29.13

0288.01328

896288.036

1663636

8

6

)1(68

6

)1(18

)(1

1

211

2

2

2

22

2

2

SolutionsGSQV = - 2.6V

DS DD D D SV = V - I R + R

= 20V - 2.6mA 4.3kΩ

= 8.82V

IDQ = 2.6mA

ID=I

S

Voltage-divider bias

A

IG=0A

Redrawn network

2G DD

1 2

RV = V

R + R

Sketching the network equation for the voltage-divider configuration.

D

GS

GS G I =0mA

GD

S V =0V

V = V

VI

R

G GS RS

GS G RS

GS G D SV

V - V - V = 0

V = V

= V - I

V

R

-

Effect of RS on the resulting Q-point.

Example : Voltage-divider bias

DQ GSQ

D

S

DS

DG

1. I andV

2. V

3. V

Determine the following for th

4. V

e netw k

5. V

or

Solutions

2G DD

1 2

DD2

RV = V

R + R

270kΩ 16V = V

2.1MΩ + 0.27MΩ

= 1.82V

D GSWhen I = 0mA, V = +1.82V

GS G D S

D

V = V - I R

= 1.82V - I 1.5kΩ

GS D

+1.82VWhen V = 0V, I = = 1.21mA

1.5kΩ

Determining the Q-point for the network

GS DV = 1.82V - I 1.5kΩ

IDQ=2.4mAVGSQ=-1.8V

DS DD SS D S D

DS S

V = V + V - I R R

= V + V = 8.82V + 2 11.6V = .42V

Mathematical solutions

How to get IDS, VGS and VDS for voltage-divider bias configuration by using mathematical solutions?

Exercise 3:

DQ GSQ

DS

D

S

1. I andV

2. V

Determine the

followi

3. V

ng for the

networ

4. V

k

Drawing the self bias line

GS D S

GS D

V + I R -10V = 0

V = 10V - I 1.5k

D GSWhen I = 0mA, V = 10V

GS D

10VWhen V = 0V, I = = 6.67mA

1.5kΩ

Determining the Q-point IDQ=6.9mAVGSQ=-0.35V DS DD SS D S DV = V - V - I R + R

= 20 +10 - (6.9mA)(1.8kΩ +1.5kΩ)

= 7.23V

D DD D DV = V - I R = 7.58V

S D DSV = V - V

= 7.58V - 7.23V = 0.35V

Exercise 4

D S

Determine the required

values of R and R

Determining VGSQ for the network.

DD DQRDD

DQ DQ

V VV 20V 12VR = =

I I 2.5mA

= 3.2k

GSQ

SDQ

V -1R = = 0.4k

I 2.5mA